It is known that conventional rigid polyurethane foam exhibits a low thermal conductivity and, therefore, is useful as a heat insulating material. However, since the rigid polyurethane foam exhibits an unsatisfactory heat resistance and flame resistance, it is impossible to utilize the conventional polyurethane foam as a heat insulating material to be used at an elevated temperature of, for example, 120.degree. C. or more.
It is also known that conventional polyisocyanurate foam exhibits a satisfactory heat resistance and flame resistance. However, conventional polyisocyanurate foam has a higher friability than that of the conventional polyurethane foam. Therefore, it was desired to reduce the friability of the polyisocyanurate foam without decreasing the heat and flame resistances thereof. For this purpose, various types of polyurethane-modified polyisocyanurate foams were provided. In the conventional polyurethan-modified polyisocyanurate foams, it was successful to reduce the friability. However, the modification of the polyisocyanurate with conventional polyurethane caused the resultant foam to exhibit a decreased heat and flame resistance. Also, the conventional modification frequently caused the smoke-generation of the resultant foam to be undesirably enhanced.
U.S. Pat. No. 3,948,824 discloses a heat resistant foam comprising a polyurethane which is a reaction product of polyisocyanate and a benzylic ether type phenolic resin. However, the heat and flame resistance of the product of the U.S. Patent was still unsatisfactory.